This invention relates to an improvement in encapsulated photoelectric measuring systems for measuring the relative position of two objects, of the type which include a measuring scale, a scanning unit for scanning the scale, means included in the scanning unit for illuminating at least a first surface of the scale, means for connecting the scanning unit with one of the two objects, a housing which encapsulates the scale and the scanning unit and defines an opening through which the connecting means passes, and at least one sealing element for sealing the opening.
Such photoelectric measuring systems are used for example in machine tools for measuring lengths or angles. These measuring systems typically must therefore be protected against environmental influences in the form of oil, cooling water, processing chips, dust and the like. One prior art approach to protection of a measuring system is shown in German Patent DE-PS No. 28 46 768. The disclosed system includes a housing for the scale and the scanning unit. This housing defines a slit running along the longitudinal direction, which slit is closed by elastomeric sealing lips. A follower extends between these sealing lips to connect the scanning unit with one of the two objects to be measured. Such sealing lips have been found to operate effectively to prevent the penetration of liquids and chips or shavings into the interior of the housing. However, such sealing lips typically cannot completely keep out liquids in the form of vapor and mist that may condense within the housing. In the event mist condenses to form liquid drops on the surfaces of the scale or the scanning plate of the scanning unit, such drops have the optical effect of collecting lenses. These drops can therefore interfere with the optical scanning of the scale and can disturb the photoelectric scanning of the graduation of the scale. Measuring inaccuracies can result from such disturbances. In addition, when a scanning unit includes printed circuits, as for example in conjunction with photosensors included in the scanning unit, condensing mist presents the danger of electrical short circuits.